1. Field of the Invention
This invention relates to sport or athletic shoes. According to an example, the shoes are constructed to minimize impact shock and to increase stability and support.
2. Description of the Related Art
In most types of footwear, especially athletic shoes, the lower or underfoot portion of the shoe includes a midsole, which is directly attached to the shoe upper portion. An outsole is attached to the midsole and is generally designed to resist wear and provide traction. The midsole is designed primarily to provide stability for the foot while attenuating shock.
When running and walking, generally the foot makes initial contact with the ground surface on the lateral portion of the rearfoot area. At initial contact, runners typically strike the ground at a force of 2.5 times their body weight, which may be repeated at a rate of 180 times per minute (90 per each foot). Therefore, the heel strike cushioning material, which is contained mostly in the midsole of a running or walking shoe must have a firmness to provide for proper impact cushioning. If the midsole material is too soft, the material will “bottom out” before heel impact is completely absorbed, and shock-related injuries could result. Softer midsoles also offer poor lateral stability that can also result in injuries.
The modern athletic shoe is a combination of elements, which cooperatively interact in an effort to minimize weight and maximize comfort, cushioning, stability and durability. However, these goals are potentially in conflict with each other and in an effort to achieve one of these objectives, a deleterious effect on one or more of the other goals can occur.
The cushioning in most athletic shoes is supplied through the foam midsole that can be made from either ethylene vinyl acetate (EVA) or polyurethane. These materials provide ample cushioning when they are new, but lose some of the cushioning ability over time due to failure of the structured materials by the application of shear and vertical forces applied to them.
A shoe industry trend has been toward thickening the midsoles of athletic shoes to enhance the cushioning effect of the sole. An added thickness of foam, however, can cause the sole to have increased stiffness in bending. Under these conditions, the lateral corner of the sole can tend to operate as a fulcrum upon heel strike and create an extended lever arm and greater moment, which can cause the foot to rotate medially and pronate with greater velocity than desired. This can lead to over-pronation of the foot and possible injury. Further, this condition can present a potentially unstable condition for the foot and result in the transmission of higher than desired levels of impact stress due to the relatively small surface area of contact.